Abstract
<jats:title>Summary</jats:title> <jats:p> <jats:list list-type="bullet"> <jats:list-item> <jats:p>Thermal performance curves (TPCs) predict species’ vulnerability to climate change, but standard respirometry assumes that measured oxygen consumption reflects physiological state. Sessile invertebrates that retract their tentacles and contract under thermal stress violate this assumption, with unmeasured consequences for thermal limit estimates.</jats:p> </jats:list-item> <jats:list-item> <jats:p> We tested this behavioral confound in an undescribed cold-water intertidal anemone ( <jats:italic>Urticina</jats:italic> sp.) in the Northwest Atlantic by integrating a negative binomial encounter-rate regression, a maximum entropy species distribution model (both from effort-corrected iNaturalist data), and closed-chamber respirometry across seven temperatures (1-30°C, 18 individuals, 126 trials). </jats:p> </jats:list-item> <jats:list-item> <jats:p>The strongest distributional predictors were cloud cover and coastal urbanization, with a weaker association with winter minimum SST; direct evidence for warm-edge thermal limitation came from the experiment. Anemone expansion state (scored 0-1 from fully closed to fully expanded) was variable and without a clear trend across the coldest treatments but declined above 20°C before collapsing at the 30°C treatment, which proved lethal to all individuals.</jats:p> </jats:list-item> <jats:list-item> <jats:p>Standard TPC models extrapolated the thermal maximum far beyond the lethal bracket (∼74°C symmetric Gaussian; 45.9°C asymmetric). A Bayesian multiplicative model that separated physiology from behavior showed that physiology continued to track temperature while expansion state declined above 20°C; a fully expanded anemone respired about twice as fast as a fully closed one at the same temperature. The decline in measured respiration is therefore both behavioral and physiological, and disentangling the two requires recording expansion state alongside oxygen consumption.</jats:p> </jats:list-item> <jats:list-item> <jats:p>Because a closed anemone cannot feed or exchange gases, the ecologically relevant thermal limit is the temperature at which the animal can no longer maintain its normal expanded posture, not a curve-fitted thermal maximum. That behavioral threshold leaves warm-edge populations within a few degrees of functional thermal failure.</jats:p> </jats:list-item> <jats:list-item> <jats:p>Future thermal physiology studies of organisms capable of modulating oxygen consumption through behavior should incorporate quantitative behavioral covariates to separate physiological from behavioral components of the metabolic response.</jats:p> </jats:list-item> </jats:list> </jats:p>